1. Field of the Invention
The present invention relates to a method of producing a glass material which is applicable to optical elements of cameras, microscopes, etc. The present invention also relates to a gel immersing apparatus which may be suitably employed to carry out the glass material producing method.
2. Description of the Related Art
In the sol-gel method, a sol is prepared by reaction of a metal alkoxide of silicon, which forms the matrix of glass, with a solvent such as ethanol, a catalyst such as hydrochloric acid, etc. When an optical glass is to be produced, various metal components are added to the sol in order to vary the refractive index. Addition of the metal components is effected by adding to the sol alcohol solutions of metal alkoxides or derivatives thereof, or solutions of metal salts.
The use of a metal salt is particularly advantageous in that the raw material is inexpensive and capable of being handled in the air. However, a gel that is obtained by using a metal salt has the disadvantage that the water and solvent in the gel volatilize when the gel shrinks on drying., causing coarse crystals of the metal salt to precipitate and grow. Since the gel is broken by the growth of the crystals, it may be impossible to obtain glass of desired configuration. Accordingly, the conventional practice is to immerse the gel in a solution in which the solubility of the metal salt is low, thereby precipitating crystallites of the metal salt in the gel. Thereafter, sintering is carried out.
Meanwhile, Japanese Patent Application Post-Examination Publication No. 2-39454 discloses a method in which a gel is immersed in an organic compound, e.g. an alcohol, dioxane, acetone, etc., in order to prevent cracking of the gel.
Incidentally, there is a distributed index optical element in which a refractive index distribution is imparted to a medium so that the medium itself has refractive power (refractive index). Since the distributed index optical element has excellent aberration correcting capability, it enables a reduction in the number of constituent lens elements. Therefore, the distributed index optical element has attracted attention as an optical element which is essential for next generation optical systems.
As methods of producing such a distributed index optical element, the sol-gel method, the ion-exchange method, the molecular stuffing method, etc. are generally employed. Particularly, the sol-gel method has the advantageous features that it is possible to obtain a glass material having a large aperture, and that a distribution can be imparted to an oxide of a multivalent metal, and further that it is possible to vary the optical characteristics of the distributed index optical element obtained. Thus, the sol-gel method is an effective production method.
As conventional methods of preparing a distributed index optical element by the sol-gel process, there have been known a method which is reported in Elect. Lett. 22 (1986), p.1108, and a method which is disclosed in U.S. Pat. No. 4,686,195.
In the method that is reported in Elect. Lett. 22 (1986), p.1108, a wet columnar gel is prepared from silicon alkoxide and germanium alkoxide or titanium alkoxide. Then, the gel is immersed in water or a dilute aqueous solution of hydrochloric acid. Germanium or titanium, which is a component that gives a high refractive index, is partly eluted by the water or dilute aqueous solution of hydrochloric acid, whereas substantially no silicon is eluted. Accordingly, only the germanium or titanium component in the gel is eluted into the water or dilute aqueous solution of hydrochloric acid. By drying and sintering the gel, it is possible to prepare a distributed index optical element in which the refractive index decreases toward the outer peripheral portion from the center.
In the method of U.S. Pat. No. 4,686,195, a solution mainly containing silicon alkoxide and a metal salt is hydrolyzed to obtain a sol, and the sol is allowed to gel. The resulting gel is immersed in a solution in which the solubility of the metal salt is low, thereby precipitating crystallites of the metal salt in the gel. The gel is then immersed in a solution containing a metal salt different from the above metal salt. Consequently, the metal salt contained in the solution gradually diffuses from the surface toward the inside of the gel. In addition, the crystallites of the metal salt contained in the gel are gradually eluted from the surface of the gel to the outside. Then, the gel is dried and sintered, thereby making it possible to prepare a distributed index optical element in which the refractive index decreases toward the outer peripheral portion from the center.
The method that is disclosed in Japanese Patent Application Post-Examination Publication No. 2-39454, and the method that is described in U.S. Pat. No. 4,686,195, that is, a method in which a gel is immersed in a solution in which the solubility of a metal salt is low, thereby precipitating crystallites of the metal salt in the gel, suffer, however, from problems as stated below. When a gel immersing treatment is actually carried out in the above-described conventional process, some portions of the meniscus or other part of the gel are likely to chip off during the immersing in the solution and disperse in the solution. The gel may crack or break into pieces even more severely. In such a case, some broken pieces of the gel are smashed into fragments by stirring, and the fragments get mixed in the treating solution in such a state that colloidal particles are dispersed in the solution, resulting in an increase in the amount of elution of metal components and organic solvent, water, acid and other components into the solution from the broken pieces of the gel and the fragments thereof. The increase in the amount of elution causes a change in the equilibrium of the solution, resulting in a change of the metal component concentration in the unbroken gel. Particularly, as the number of gels which are treated simultaneously in a single container increases, the gels are more affected by the eluted substances and the fragments of the gel, thus producing a serious adverse effect on the yield. When such a gel is used to impart a refractive index distribution thereto, there may be a variation in the distribution profile because there is a variation in the metal concentration distribution in the gel.
Further, since the dispersed fragments of the gel act as a nucleating agent, coarse crystals of the metal salt are likely to form in some portions of the gel, causing the gel to break. The fragments of the gel may also cause crystallization on sintering, resulting in a white opaque bulk material being formed. Therefore, it has heretofore been difficult to obtain a crack-free transparent glass material.
There is a fixation and distribution imparting process in which a single gel is put in a single container to precipitate crystallites of a metal salt in the gel. This fixation and distribution imparting process requires a large number of steps and facilities, and it is therefore inefficient. Accordingly, it is essential from the industrial point of view to employ an efficient production method in which a plurality of gels are simultaneously put in a single container to carry out fixation and distribution imparting operations. In a method wherein a plurality of gels are simultaneously treated, if an extremely large amount of solution is used to treat the gels, for example, the above-described change in the concentration of the solution reduces. Therefore, the adverse effect on the yield will reduce. However, this method causes an increase in the amount of solution used and also an increase in size of the system, and hence unfavorably requires a wider space and a great deal of cost. Accordingly, it has been demanded to establish a glass producing method which requires a small amount of solution for treatment, and which enables a plurality of gels to be simultaneously and readily treated in a short time by using a small-sized system. More particularly, it has been demanded to establish a glass producing method which enables a glass material stabilized in quality, e.g. refractive index, to be readily obtained in a short time at a reduced cost.
In a case where a distributed index optical element is used in a lens system of a camera or the like, the refractive index distribution profile is important. The following expression shows the relationship between the refractive index and the distance from the center of a distributed index optical element: EQU N(r)=N.sub.0 +N.sub.1 r.sup.2 +N.sub.2 r.sup.4 +N.sub.3 r.sup.6 +. . .
where N(r) is the refractive index at a point defined by a radius r from the center, N.sub.0 is the refractive index at the center, and N.sub.1, N.sub.2, N.sub.3. . . are distribution coefficients.
Matters of great concern in terms of the distribution profile are the refractive index difference .DELTA.n between the central and peripheral portions, and the distribution coefficient at the lens peripheral portion for correcting aberrations. The power of a distributed index optical element is determined by the value of N.sub.1. Therefore, as the absolute value of N.sub.1 increases, that is, as .DELTA.n increases, the effect of the distributed index optical element becomes more powerful. For the aberration correction at the lens peripheral portion, the distribution coefficients N.sub.2, N.sub.3. . ., which have a great effect on the distribution profile at the peripheral portion are important factors. It is generally known that a glass material which has a large refractive index difference and a parabolic refractive index distribution profile is useful from the viewpoint of aberration correcting capability and the readiness of lens design. Therefore, it has heretofore been considered necessary to establish a method of producing such a glass material.
However, neither of the methods disclosed in Elect. Lett. 22 (1986), p.1108 and U.S. Pat. No. 4,686,195 can produce the desired glass material. That is, the glass material produced by either of the conventional methods has an unfavorably small refractive index difference and a non-parabolic refractive index distribution profile, that is, a distribution profile in which the refractive index distribution curve is undesirably inflected at the lens peripheral portion. FIG. 10 is a conceptual view showing the refractive index distribution. In the figure, the dotted line shows a parabolic distribution profile, and the solid line shows a distribution profile obtained by the above-described prior art. When a lens system is prepared by using a glass material having such a non-parabolic distribution profile, since the value of An is small, the power of the medium itself is weak, and since the distribution curve has a point of inflection at the lens peripheral portion, light rays cannot effectively be converged. Accordingly, aberration correcting effect which is unique to the distributed index optical element cannot satisfactorily be obtained. If the glass peripheral portion is cut off so as to avoid the point of inflection at the peripheral portion, the effective aperture of the lens is undesirably limited.
The mechanism of the occurrence of a disorder in the distribution profile, which is a problem associated with the prior art, will be explained below by way of an example shown in U.S. Pat. No. 4,686,195. In the example, attempts were made to control the metal concentration distribution profile, that is, the refractive index distribution, by changing the time for which a gel containing lead acetate was immersed in a distribution imparting solution, to thereby control the amount of elution of lead acetate.
FIG. 11 shows the time for which the gel was immersed in the distribution imparting solution and the profile of the metal concentration in the gel. The gel was immersed in the distribution imparting solution for three different periods of time: 4 hours (characteristic curve a); 16 hours (characteristics curve b); and 24 hours (characteristic curve c). It will be understood from FIG. 11 that, as the immersing time becomes longer, the metal concentration at the gel peripheral portion increases, and the parabolic distribution profile is gradually deformed. As a result, it becomes impossible to obtain a gel having the desired parabolic lead concentration distribution. FIG. 12 shows the refractive index distributions of glass materials obtained by sintering the gels prepared as described above. In the figure, the characteristic curve a shows the refractive index distribution of a glass material produced from a gel immersed for 4 hours in the same way as in FIG. 11. The characteristic curve b shows the refractive index distribution of a glass material produced from a gel immersed for 16 hours. The characteristic curve c shows the refractive index distribution of a glass material produced from a gel immersed for 24 hours. As will be understood from FIG. 12, the parabolic distribution profile is deformed, resulting in a glass material having a refractive index distribution profile in which the distribution curve is undesirably inflected at the outer peripheral portion, as shown in the conceptual view of FIG. 10.
The above-described phenomenon may be explained as follows: It may be considered that, when a columnar gel containing a salt of metal species A is immersed in a distribution imparting solution containing a salt of metal species B, the formation of a radial concentration distribution of each of the metal species A and B during the distribution imparting process takes place according to the diffusion rule. More specifically, metal ions A diffuse into the distribution imparting solution having a relatively low concentration of metal species A from the gel having a relatively high concentration of metal species A. Conversely, the metal species B in the distribution imparting solution diffuse into the gel having a low concentration of metal species B from the solution having a relatively high concentration of metal species B. The reaction taking place at that time may be considered to be an ion-exchange reaction, and upon completion of the distribution impartation, the concentration gradients of metal species A and B from the central portion of the gel toward the peripheral portion thereof must have no point of inflection.
In actual practice, however, the metal species A in the gel are eluted into the distribution imparting solution. Therefore, the A concentration in the dipping solution increases, and thus the difference in the A concentration between the gel and the distribution imparting solution decreases. As a result, the dissolution equilibrium changes. Accordingly, the rate of dissolution of the metal species in the distribution imparting solution gradually reduces. The degree of reduction in the dissolution rate differs depending on the kind of metal species eluted from the gel and the kind of solution. In many cases, however, the dissolution rate reduces as shown in FIG. 13. That is, when a certain amount of metal species A has been eluted into the solution, the rate of dissolution reduces. As the rate of dissolution reduces, the elution of the metal species A from the gel becomes slow, and the time required for the metal species A to be eluted increases, resulting in a metal concentration distribution in which the rate of elution of the metal species A from the gel into the solution gradually reduces. That is, no parabolic distribution profile such as that shown by the dotted line in FIG. 10 can be obtained, and the resulting distribution profile has a point of inflection such that the distribution curve becomes undesirably gentle at the outer peripheral portion of the gel, as shown by the solid line in the figure.
Further, as the dipping time lengthens, the metal species B in the solution diffuse into pores of the gel. Consequently, the concentration of the metal species B in the solution gradually reduces. In other words, since the rate of diffusion of the metal species B to be supplied into the gel decreases, no ideal diffusion takes place, and thus the distribution profile is deformed.
Thus, since the distribution imparting conditions are greatly affected by the presence of metal ions eluted into the distribution imparting solution from the gel and metal ions diffused into the gel from the solution, a long time is required for the distribution impartation, and it is impossible to obtain a distributed index optical element having a parabolic distribution profile and a large refractive index difference.
Although in the forgoing the fixation of crystallites, the distribution impartation, etc., which are related to the chemical equilibrium of a solution have been described, it should be noted that an adverse effect is also produced when dust or the like, which is not related to the chemical equilibrium, is attached to the gel. For example, when dust is attached to the gel in the distribution imparting process, the way in which the distribution imparting solution contacts the gel changes. Therefore, it becomes impossible to impart a uniform distribution. Thus, the distribution is disordered.
Further, since foreign substances such as fragments of the gel act as a nucleating agent and cause crystallization, as described above, the resulting glass material may become opaque. For example, metal components such as Ti, Ba, Sn, Zr, Cu, and Nb in particular are likely to act as a nucleating agent, and when attached to the gel, these metal components cause crystallization and devitrification during sintering process.
Thus, with the prior art, the amount of fixation of a metal component to a gel and the amount of elution of a metal component from the gel are affected by the solubility in the solution, that is, the amount and composition of the solution (the kinds of metal, solvent and acid contained in the solution, and the concentrations thereof) and the existence of dust or other impurities. Unless these components and impurities are removed and the relationship between the gel and the dipping solution is kept constant at all times, the amount of metal component fixed in the gel varies, and the distribution rate changes, resulting in variation of the refractive index distribution profile. Consequently, it is impossible to obtain a glass material having a large refractive index difference and a parabolic distribution profile. Moreover, it is impossible to obtain an optical element stabilized in quality.